Hitherto, various studies have been made on filter materials using a nonwoven fabric (Patent Document 1). However, the conventional filter materials using a nonwoven fabric tend to have such a contradictory problem that when it is intended to increase a filtering accuracy of the filters, there occur a high pressure loss and a shortened filtering life thereof, whereas it is intended to reduce the pressure loss, the filtering accuracy is deteriorated.
More specifically, in the nonwoven fabric-based filter materials, the filtering performance may vary depending upon diameter or diameter distribution of fibers used therein. Therefore, when melt-blown (MB) fibers or spun-bonded (SB) fibers which exhibit a large variation of fineness in a fiber diameter range of 1 μm or more are used for the filter materials, it will be difficult to apply filters obtained therefrom to microfiltration or ultrafiltration in which the filters are required to have a capability of collecting fine particles having a particle size in the order of μm. In order to further increase the filtering accuracy, the filter materials must be subjected to calendering treatment, etc., to increase a fiber density thereof, which also results in occurrence of the above problems.
Also, glass fibers are capable of providing a sheet-like filter material formed of the fibers having a fiber diameter of 1 μm or less. Although the sheet-like filter material formed of the glass fibers exhibits a high collecting efficiency, there tends to occur such a problem that a binder component contained therein is eluted out depending upon use conditions thereof. Further, the filter material formed of the glass fibers tends to suffer from falling-off of the fibers therefrom.
On the other hand, there has been proposed the high-precision membrane filters having a very high filtering accuracy in which a standard deviation of pore sizes thereof is 0.1 μm or less (Patent Documents 2 and 3). However, these membrane filters have problems such as occurrence of early clogging and shortened filtering life as compared to the above nonwoven fabric-based filters.
Patent Document 1: JP 3449429
Patent Document 2: PCT Pamphlet WO 2002/064240
Patent Document 3: JP 2008-229612A